Spectrally shaped tunable short-cavity laser

ABSTRACT

A tunable source includes a short-cavity laser optimized for performance and reliability in SSOCT imaging systems, spectroscopic detection systems, and other types of detection and sensing systems. The short cavity laser has a large free spectral range cavity, fast tuning response and single transverse, longitudinal and polarization mode operation, and includes embodiments for fast and wide tuning, and optimized spectral shaping. Disclosed are both electrical and optical pumping in a MEMS-VCSEL geometry with mirror and gain regions optimized for wide tuning, high output power, and a variety of preferred wavelength ranges; and a semiconductor optical amplifier, combined with the short-cavity laser to produce high-power, spectrally shaped operation. Several preferred imaging and detection systems make use of this tunable source for optimized operation are also disclosed.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/600,074, filed on May 19, 2017, which is a continuation of U.S.patent application Ser. No. 13/952,554, filed on Jul. 26, 2013 andpatented on Dec. 12, 2017 as U.S. Pat. No. 9,843,159, which claims thebenefit of U.S. Provisional Patent Application No. 61/676,712 filed onJul. 27, 2012. The disclosures of U.S. patent application Ser. No.15/600,074, U.S. patent application Ser. No. 13/952,554 and U.S.Provisional Patent Application 61/676,712 are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to tunable lasers, widely tunable lasers,wavelength swept sources, amplified tunable lasers, rapidly tunedlasers, and optical systems enabled by these devices.

BACKGROUND

Widely and rapidly tunable lasers are important for a variety ofdetection, communication, measurement, therapeutic, sample modification,and imaging systems. For example, swept source optical coherencetomography (SSOCT) systems employ repetitively swept tunable lasers togenerate subsurface microstructural images of a wide range of materials.In SS-OCT, wide tuning range translates to higher axial measurementresolution, and higher tuning speed enables real-time acquisition oflarge data sets. In addition, variable tuning speed enables trading offimaging range and resolution as required for different applications.Lastly, long coherence length, which is equivalent to narrow linewidth,enables long imaging range. Another example of a system which requiresrapidly and widely tunable lasers is transient gas spectroscopy as, forexample, described in (Stein, B. A., Jayaraman, V. Jiang, J. J, et al.,“Doppler-limited H20 and HF absorption spectroscopy by sweeping the1321-1354 nm range at 55 kHz repetition rate using a single-modeMEMS-tunable VCSEL,” Applied Physics B: Lasers and Optics 108(4), 721-5(2012)). In gas spectroscopy, tuning speed enables characterization oftime-varying processes, such as in engine thermometry. Narrow spectralwidth enables resolution of narrow absorption features, such as thosethat occur at low gas temperatures. Other transient spectroscopicapplications include monitoring of explosive or other non-repetitiveprocesses.

Beyond wide tunability and long coherence length, other importantparameters for tunable lasers for a variety of applications includetuning speed and variability of tuning speed. In SS-OCT, increasedtuning speed enables imaging of time-varying physiological processes, aswell as real-time volumetric imaging of larger data sets. Also forSS-OCT, variability of tuning speed enables switching between highspeed, high resolution short-range imaging, and low speed, lowresolution long range imaging in a single device, which is of greatutility in, for example, ophthalmic imaging, as described in(Grulkowski, I., Liu, J. J., Potsaid, B. et al., “Retinal, anteriorsegment and full eye imaging using ultrahigh speed swept source OCTwithvertical-cavity surface emitting lasers,” Biomed. Opt. Express, 3(11),2733-2751 (2012)). Spectroscopic or other detection applications benefitin analogous ways from high-speed and variable speed.

Further desirable properties of widely tunable lasers include highoutput power, center wavelength flexibility, spectrally shaped output,monolithic and low-cost fabrication, and compatibility with arraytechnology. High power increases signal to noise ratio for virtuallyevery application. Center wavelength flexibility translates into greaterutility in a larger variety of applications. Spectrally shaped outputalso increases signal to noise ratio and improves thermal management.Monolithic, low cost fabrication has obvious advantages, and arraytechnology simplifies applications in which multiple sources aremultiplexed.

The limitations of prior art tunable lasers with respect to thedesirable properties above can be understood by examination of threerepresentative examples. These examples include Fourier Domainmode-locked (FDML) lasers, external cavity tunable lasers (ECTL), andsampled grating distributed bragg reflector (SGDBR) lasers. An FDMLlaser is described in (Huber, R., Adler, D. C., and Fujimoto, J. G.,“Buffered Fourier domain mode locking: unidirectional swept lasersources for optical coherence tomography imaging at 370,000 lines/s,”Optics Letters, 31(20), 2975-2977 (2006)). Use of a commercial ECTL inan SSOCT system is described in (George, B., Potsaid, B., Baumann, B.,Huang, D. et al., “Ultrahigh speed 1050 nm swept source/Fourier domainOCT retinal and anterior segment imaging at 100,000 to 400,000 axialscans per second,” Optics Express, 18(19), 20029-20048 (2010)).Operation of an SGDBR laser is described in (Derickson, D., “High-SpeedConcatenation of Frequency Ramps Using Sampled Grating Distributed BraggReflector Laser Diode Sources for OCT Resolution Enhancement,”Proceedings of the SPIE—The International Society for OpticalEngineering, 7554, (2010)). FDML and ECTL devices are essentiallymulti-longitudinal mode devices, which sweep a cluster of modes insteadof a single mode across a tuning range. This results in limited imagingrange for SSOCT and limited spectral resolution for spectroscopicapplications. Both FDML and ECTL are also non-monolithic sources, whichare assembled from discrete components, and therefore not low costdevices or compatible with array fabrication. The ECTL further suffersfrom fundamental speed limitations of about 100 kHz repetition rate orless, due to the long time delay in the external cavity, as described in(Huber, R., Wojtkowski, M., Taira, K. et al., “Amplified, frequencyswept lasers for frequency domain reflectometry and OCT imaging: designand scaling principles,” Optics Express, 13(9), 3513-3528 (2005).)Further speed limitations in ECTL devices arise from the large mass ofthe grating tuning element, as for example in the commercially availableThorlabs model SL1325-P16 grating tuned laser. The FDML suffers alsofrom inflexiblity of both center wavelength and tuning speed. Since theFDML employs a long fiber-based cavity, it can only operate atwavelengths where low-loss optical fiber is readily available. Secondly,the FDML sweep rate is fixed by the roundtrip time of light in the fiberexternal cavity, and variable sweep rates are therefore not possible ina single devices.

The SGDBR is a single transverse and longitudinal mode device, and hasthe potential for long imaging range and narrow spectral width. Tuning,however, is accomplished by discontinuous hopping amongst various modes,which tends to introduce measurement artifacts. The mode-hopping alsorequires multiple tuning electrodes, complicated drive circuitry andassociated speed limitations. The SGDBR also suffers from limited tuningrange relative to external cavity and FDML lasers, since the latter uselossless tuning mechanisms, while the SGDBR is tuned by free carrierinjection, which introduces free carrier losses and limits tuning range.The SGDBR also suffers from center wavelength inflexibility, due to theneed for complex regrowth fabrication technology which is only mature inthe Indium Phosphide material system.

The problems discussed above with respect to the FDML, ECTL, and SGDBRabove are representative of problems encountered by most tunable lasersknown in the art.

MEMS-tunable vertical cavity lasers (MEMS-VCSELs) offer a potentialsolution to the problems above. The short cavity of MEMS-VCSELs leads toa large longitudinal mode spacing and relative immunity to mode hops.The MEMS-VCSEL requires only one tuning electrode to sweep a single modeacross the tuning range, and therefore offers the promise of long SS-OCTimaging range with minimal measurement artifacts, and rapid tuning. Theshort cavity and the short mass of the MEMS mirror offer the potentialfor very high speed. MEMS-VCSEL technology can also be extended to alarge variety of wavelength ranges difficult to access with many othertypes of sources, making them appropriate for other types ofspectroscopic, diagnostic, and detection systems. The application ofMEMS-VCSELs to SS-OCT imaging was first described in U.S. Pat. No.7,468,997. MEMS-VCSELs have the potential for wide tuning range, asdiscussed in U.S. Pat. No. 7,468,997. Until 2011, however, the widestMEMS-VCSEL tuning range achieved was 65 nm around 1550 nm, as describedin (Matsui, Y., Vakhshoori, D., Peidong, W. et al., “Completepolarization mode control of long-wavelength tunable vertical-cavitysurface-emitting lasers over 65-nm tuning, up to 14-mW output power,”IEEE Journal of Quantum Electronics, 39(9), 1037-10481048 (2003). Thisrepresents a fractional tuning range of about 4.2%, or about a factor of2 less than that required in SS-OCT imaging.) In 2011, a tuning range of111 nm was demonstrated in a 1310 nm MEMS-VCSEL, which was subsequentlyapplied in an SSOCT imaging system, as described in (Jayaraman, V.,Jiang, J., Li, H. et al., “OCT Imaging up to 760 kHz Axial Scan RateUsing Single-Mode 1310 nm MEMS-Tunable VCSELs with >100 nm TuningRange,” CLEO: 2011—Laser Science to Photonic Applications, 2 pp.-2 pp. 2pp. (2011).)

The MEMS-VCSEL described by Jayaraman, et al. in 2011 represented amajor innovation in widely tunable short cavity lasers. Achievingperformance and reliability appropriate for commercial optical systems,however, requires optimization of tuning speed, frequency response oftuning, tuning range, spectral shape of tuning curve, output power vs.wavelength, post-amplified performance, gain and mirror designs, andoverall cavity design. Numerous design innovations are required toimprove upon the prior art to achieve performance and reliabilitynecessary for these commercial systems.

From the foregoing, it is clear that what is required is a widelytunable short-cavity laser with 3-dimensional cavity and material designoptimized for performance and reliability in SSOCT imaging systems,spectroscopic detection systems and other types of optical systems.

SUMMARY

This document provides several preferred embodiments of a tunable sourcecomprising a short-cavity laser optimized for performance andreliability in SSOCT imaging systems, spectroscopic detection systems,and other types of detection and sensing systems. This document presentsa short cavity laser with a large free spectral range cavity, fasttuning response and single transverse, longitudinal and polarizationmode operation. The disclosure includes embodiments for fast and widetuning, and optimized spectral shaping. Preferred embodiments includeboth electrical and optical pumping in a MEMS-VCSEL geometry with mirrorand gain regions optimized for wide tuning, high output power, and avariety of preferred wavelength ranges. Other preferred embodimentsinclude a semiconductor optical amplifier, combined with theshort-cavity laser to produce high-power, spectrally shaped operation.Several preferred imaging and detection system embodiments make use ofthis tunable source for optimized operation.

One embodiment provides a tunable laser operative to emit tunableradiation over an emission wavelength range having a center wavelength,with an output power spectrum over said wavelength range and an averageemission power, said tunable laser comprising: an optical cavityincluding a first and second mirror; a gain region interposed betweensaid first and second mirrors; a tuning region; and means for adjustingan optical path length of said tuning region; wherein: a free spectralrange (FSR) of said optical cavity exceeds 5% of said center wavelength;said tunable laser operates substantially in a single longitudinal andtransverse mode over said wavelength range; and said means for adjustingan optical path length has a wavelength tuning frequency response with a6-dB bandwidth greater than about 1 kHz.

One embodiment provides an amplified tunable laser source, comprisinginput tunable radiation with an input power spectrum, an input centerwavelength, input wavelength range and input average power, opticallycoupled to an input side of a semiconductor optical amplifier comprisingat least one quantum well, and output tunable radiation having an outputpower spectrum, an output center wavelength, an output wavelength range,and output average power, emerging from an output side of said opticalamplifier, wherein said input tunable radiation is generated by atunable laser comprising: an optical cavity including a first and secondmirror; a gain region interposed between said first and second mirrors;a tuning region; and means for adjusting an optical path length of saidtuning region; wherein: a free spectral range (FSR) of said opticalcavity exceeds 5% of said center wavelength; said tunable laser operatessubstantially in a single longitudinal and transverse mode over saidwavelength range; and said means for adjusting an optical path lengthhas a wavelength tuning frequency response with a 6-dB bandwidth greaterthan about 1 kHz.

Another embodiment provides an amplified tunable laser source,comprising input tunable radiation with an input power spectrum, aninput center wavelength, input wavelength range and input average power,optically coupled to an input side of a fiber-based optical amplifiercomprising at least one quantum well, and output tunable radiationhaving an output power spectrum, an output center wavelength, an outputwavelength range, and output average power, emerging from an output sideof said optical amplifier, wherein said input tunable radiation isgenerated by a tunable laser comprising: an optical cavity including afirst and second mirror; a gain region interposed between said first andsecond mirrors; a tuning region; and means for adjusting an optical pathlength of said tuning region; wherein: a free spectral range (FSR) ofsaid optical cavity exceeds 5% of said center wavelength; said tunablelaser operates substantially in a single longitudinal and transversemode over said wavelength range; and said means for adjusting an opticalpath length has a wavelength tuning frequency response with a 6-dBbandwidth greater than about 1 kHz.

One embodiment provides a tunable laser operative to emit tunableradiation over an emission wavelength range having a center wavelength,with an output power spectrum over said wavelength range and an averageemission power, said tunable laser comprising: an optical cavityincluding a first and second mirror; a gain region interposed betweensaid first and second mirrors; an airgap tuning region; and aMEMS-actuation mechanism for adjusting said airgap, said actuationmechanism comprising a deformable dielectric membrane transparent oversaid wavelength range and attached to a rigid support structure;wherein: a free spectral range (FSR) of said optical cavity exceeds 5%of said center wavelength; said tunable laser operates substantially ina single longitudinal and transverse mode over said wavelength range;and said MEMS-actuation mechanism has a wavelength tuning frequencyresponse with a 6-dB bandwidth greater than about 1 kHz.

One embodiment provides a tunable laser operative to emit tunableradiation over an emission wavelength range having a center wavelength,with an output power spectrum over said wavelength range and an averageemission power, said tunable laser comprising: an optical cavityincluding a first and second mirror; a quantum well gain regioninterposed between said first and second mirrors and comprising at leastone quantum well; a tuning region; and means for adjusting an opticalpath length of said tuning region; wherein: a free spectral range (FSR)of said optical cavity exceeds 5% of said center wavelength; saidtunable laser operates substantially in a single longitudinal andtransverse mode over said wavelength range; said means for adjusting anoptical path length has a wavelength tuning frequency response with a6-dB bandwidth greater than about 1 kHz, and each of said at least atleast one quantum well is substantially aligned with a peak in anoptical standing wave pattern of said optical cavity.

One embodiment provides a tunable laser operative to emit tunableradiation over an emission wavelength range having a center wavelength,with an output power spectrum over said wavelength range and an averageemission power, said tunable laser comprising: an optical cavityincluding a first and second mirror; a gain region interposed betweensaid first and second mirrors; a tuning region; means for adjusting anoptical path length of said tuning region; and means to achieve asubstantially single polarization state over said wavelength range;wherein: a free spectral range (FSR) of said optical cavity exceeds 5%of said center wavelength; said tunable laser operates substantially ina single longitudinal and transverse mode over said wavelength range;and said means for adjusting an optical path length has a wavelengthtuning frequency response with a 6-dB bandwidth greater than about 1kHz.

One embodiment provides an optical system for spectroscopic probing of asample, said system comprising: a tunable laser; and means fordetection; wherein said tunable laser is operative to emit tunableradiation over an emission wavelength range having a center wavelength,with an output power spectrum over said wavelength range and an averageemission power, said tunable laser comprising: an optical cavityincluding a first and second mirror; a gain region interposed betweensaid first and second mirrors; a tuning region; and means for adjustingan optical path length of said tuning region; wherein: a free spectralrange (FSR) of said optical cavity exceeds 5% of said center wavelength;said tunable laser operates substantially in a single longitudinal andtransverse mode over said wavelength range; and said means for adjustingan optical path length has a wavelength tuning frequency response with a6-dB bandwidth greater than about 1 kHz.

Another embodiment provides a system for optical beam steering, thesystem comprising: a tunable laser; and means to convert wavelengthvariation into beam deflection; wherein said tunable laser is operativeto emit tunable radiation over an emission wavelength range having acenter wavelength, with an output power spectrum over said wavelengthrange and an average emission power, said tunable laser comprising: anoptical cavity including a first and second mirror; a gain regioninterposed between said first and second mirrors; a tuning region; andmeans for adjusting an optical path length of said tuning region;wherein: a free spectral range (FSR) of said optical cavity exceeds 5%of said center wavelength; said tunable laser operates substantially ina single longitudinal and transverse mode over said wavelength range;and said means for adjusting an optical path length has a wavelengthtuning frequency response with a 6-dB bandwidth greater than about 1kHz.

Another embodiment provides a rapidly tuned oscillator, comprising: atunable laser; a second laser; and means for generating a beat signalbetween radiation emerging from the tunable laser and radiation emergingfrom said second laser; wherein said tunable laser is operative to emittunable radiation over an emission wavelength range having a centerwavelength, with an output power spectrum over said wavelength range andan average emission power, said tunable laser comprising: an opticalcavity including a first and second mirror; a gain region interposedbetween said first and second mirrors; a tuning region; and means foradjusting an optical path length of said tuning region; wherein: a freespectral range (FSR) of said optical cavity exceeds 5% of said centerwavelength; said tunable laser operates substantially in a singlelongitudinal and transverse mode over said wavelength range; and saidmeans has a wavelength tuning frequency response with a 6-dB bandwidthgreater than about 1 kHz.

Another embodiment provides an array of tunable lasers on a commonwafer, wherein each of the tunable lasers is operative to emit tunableradiation over an emission wavelength range having a center wavelength,with an output power spectrum over said wavelength range and an averageemission power, said each of the tunable lasers comprising: an opticalcavity including a first and second mirror; a gain region interposedbetween said first and second mirrors; a tuning region; and means foradjusting an optical path length of said tuning region; wherein: a freespectral range (FSR) of said optical cavity exceeds 5% of said centerwavelength; said tunable laser operates substantially in a singlelongitudinal and transverse mode over said wavelength range; and saidmeans for adjusting an optical path length has a wavelength tuningfrequency response with a 6-dB bandwidth greater than about 1 kHz.

Another embodiment provides a high speed swept source comprising atunable laser having a wavelength tuning range less than 50% of saidFSR, and means for interleaving at least one copy of said wavelengthsweep with the original sweep to generate a multiplied effective sweeprate; wherein the tunable laser is operative to emit tunable radiationover an emission wavelength range having a center wavelength, with anoutput power spectrum over said wavelength range and an average emissionpower, said tunable laser comprising: an optical cavity including afirst and second mirror; a gain region interposed between said first andsecond mirrors; a tuning region; and means for adjusting an optical pathlength of said tuning region; wherein: a free spectral range (FSR) ofsaid optical cavity exceeds 5% of said center wavelength; said tunablelaser operates substantially in a single longitudinal and transversemode over said wavelength range; and said means has a wavelength tuningfrequency response with a 6-dB bandwidth greater than about 1 kHz.

Another embodiment provides a high speed swept source comprising a firsttunable laser and a second tunable laser, wherein each of the first andsecond tunable laser is operative to emit tunable radiation over anemission wavelength range having a center wavelength, with an outputpower spectrum over said wavelength range and an average emission power,said tunable laser comprising: an optical cavity including a first andsecond mirror; a gain region interposed between said first and secondmirrors; a tuning region; and means for adjusting an optical path lengthof said tuning region; wherein: a free spectral range (FSR) of saidoptical cavity exceeds 5% of said center wavelength; said tunable laseroperates substantially in a single longitudinal and transverse mode oversaid wavelength range; and said means has a wavelength tuning frequencyresponse with a 6-dB bandwidth greater than about 1 kHz; and wherein afirst wavelength sweep of said first tunable laser and a secondwavelength sweep of said second tunable laser are interleaved togenerate a multiplied effective sweep rate.

Another embodiment provides a method for generating a desired outputpower variation vs. wavelength over a wavelength range from a tunablelaser operative to emit tunable radiation over an emission wavelengthrange having a center wavelength, with an output power spectrum oversaid wavelength range and an average emission power, said tunable lasercomprising: an optical cavity including a first and second mirror; again region interposed between said first and second mirrors; a tuningregion; and means for adjusting an optical path length of said tuningregion; wherein: a free spectral range (FSR) of said optical cavityexceeds 5% of said center wavelength; said tunable laser operatessubstantially in a single longitudinal and transverse mode over saidwavelength range; and said means for adjusting an optical path lengthhas a wavelength tuning frequency response with a 6-dB bandwidth greaterthan about 1 kHz, the method comprising defining a target powervariation over the wavelength range, and adjusting a reflectivityspectrum of one of said first and second mirrors until said output powervariation matches said target power variation over the wavelength range.

Another embodiment provides a method for generating a desired outputpower variation vs. wavelength over a wavelength range from a tunablelaser operative to emit tunable radiation over an emission wavelengthrange having a center wavelength, with an output power spectrum oversaid wavelength range and an average emission power, said tunable lasercomprising: an optical cavity including a first and second mirror; again region interposed between said first and second mirrors; a tuningregion; and means for adjusting an optical path length of said tuningregion; wherein: a free spectral range (FSR) of said optical cavityexceeds 5% of said center wavelength; said tunable laser operatessubstantially in a single longitudinal and transverse mode over saidwavelength range; and said means for adjusting an optical path lengthhas a wavelength tuning frequency response with a 6-dB bandwidth greaterthan about 1 kHz, the method comprising shaping a pump energy enteringinto the gain region vs. wavelength to achieve a target output powervariation vs. wavelength over the wavelength range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of widely tunable short cavity laseraccording to an embodiment.

FIG. 2 illustrates an output power spectrum of a widely tunableshort-cavity laser.

FIG. 3 illustrates the definition of free spectral range.

FIG. 4 shows a water vapor absorption spectrum in the 1330-1365 nm rang.

FIG. 5 shows a measurement dynamic coherence length obtained by therolloff of the OCT point spread function vs. imaging depth.

FIG. 6 illustrates an embodiment of a widely tunable short-cavity laserwith closed loop control.

FIG. 7 illustrates a MEMS-VCSEL implementation of a tunable short cavitylaser operating near 1310 nm.

FIG. 8 illustrates an axial refractive index profile of a short cavitylaser having 4 standing wave maxima between two mirrors of the cavity.

FIG. 9 illustrates the static and dynamic tuning response of theMEMS-VCSEL illustrated in FIG. 7.

FIG. 10 illustrates a variety of MEMS-VCSEL actuator frequencyresponses.

FIG. 11 illustrates a widely tunable short-cavity laser with piezotuning.

FIG. 12 illustrates various actuator geometries.

FIG. 13 illustrates oxidation for a fully oxidized mirror proceedingfrom two etched holes.

FIG. 14 illustrates a 1310 nm reflectivity spectrum configured tosupport pumping at 1050 nm.

FIG. 15 illustrates an embodiment of a widely tunable short cavity lasercoupled to an optical amplifier.

FIG. 16 illustrates an ASE spectrum from a dual-quantum statesemiconductor optical amplifier.

FIG. 17 illustrates a widely tunable short cavity laser coupled to anoptical amplifier, the output of which is coupled to a synchronouslytuned optical filter.

FIG. 18 illustrates an amplified widely tunable short cavity laser witha tunable optical filter between the laser and amplifier.

FIG. 19 illustrates an embodiment of a widely tunable short cavity laserwith two amplification stages.

FIG. 20 illustrates an embodiment of a widely tunable short cavity laserwith two amplification stages and a tunable optical filter between thestages.

FIG. 21 illustrates an amplified and pre-amplified spectrum of a widelytunable short cavity laser.

FIG. 22 illustrates various output power spectra of a widely tunableshort-cavity laser operating near 1310 nm.

FIG. 23 illustrates a MEMS-VCSEL implementation of a widely tunableshort-cavity laser operating near 1060 nm.

FIG. 24 illustrates static and dynamic tuning spectra of the MEMS-VCSELin FIG. 23.

FIG. 25 illustrates steps 1-4 in the fabrication of a widely tunableshort cavity laser realized as a MEMS-VCSEL.

FIG. 26 illustrates steps 5-6 in the fabrication of a widely tunableshort cavity laser realized as a MEMS-VCSEL.

FIG. 27 illustrates an electrically pumped MEMS-VCSEL implementation ofa widely tunable short-cavity laser.

FIG. 28 illustrates wavelength sweeps from two short-cavity lasersinterleaved to produce a multiplied sweep rate.

DETAILED DESCRIPTION

The description of illustrative embodiments according to principles ofthe present invention is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments of the inventiondisclosed herein, any reference to direction or orientation is merelyintended for convenience of description and is not intended in any wayto limit the scope of the present invention. Relative terms such as“lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,”“down,” “top” and “bottom” as well as derivative thereof (e.g.,“horizontally,” “downwardly,” “upwardly,” etc.) should be construed torefer to the orientation as then described or as shown in the drawingunder discussion. These relative terms are for convenience ofdescription only and do not require that the apparatus be constructed oroperated in a particular orientation unless explicitly indicated assuch. Terms such as “attached,” “affixed,” “connected,” “coupled,”“interconnected,” and similar refer to a relationship wherein structuresare secured or attached to one another either directly or indirectlythrough intervening structures, as well as both movable or rigidattachments or relationships, unless expressly described otherwise.Moreover, the features and benefits of the invention are illustrated byreference to the exemplified embodiments. Accordingly, the inventionexpressly should not be limited to such exemplary embodimentsillustrating some possible non-limiting combination of features that mayexist alone or in other combinations of features; the scope of theinvention being defined by the claims appended hereto.

This disclosure describes the best mode or modes of practicing theinvention as presently contemplated. This description is not intended tobe understood in a limiting sense, but provides an example of theinvention presented solely for illustrative purposes by reference to theaccompanying drawings to advise one of ordinary skill in the art of theadvantages and construction of the invention. In the various views ofthe drawings, like reference characters designate like or similar parts.

FIGS. 1-3 illustrate properties of a preferred embodiment of ashort-cavity tunable laser in accordance with the present invention. Asshown in FIG. 1, the laser 100 comprises a gain region 110 and a tuningregion 120, interposed between a first mirror 130 and a second mirror140. Energy to support lasing operation can be supplied to the gainregion in the form of optical or electrical pumping, as is well-known tothose skilled in the art of lasers. A thermally conductive heatspreading layer 150, preferably a metal such as gold, gold-tin, indium,or indium containing solder adjacent one mirror can also be employed toincrease an average output power of the tunable short cavity laser. Inthe case of a vertical cavity laser on a GaAs substrate, for example, asubstrate via could be etched, stopping on the second mirror, on whichthe heat-spreading layer could be deposited through the substrate via.

Referring to FIG. 1, adjustment of the effective optical path length ofthe tuning region causes the wavelength of the laser to be tuned. Thelaser emits wavelength tunable radiation, which is emitted through thefirst mirror. A typical emitted power spectrum 200, which is the poweremitted as a function of wavelength, as shown in FIG. 2. The Spectrumrepresents range of wavelengths in tunable emission and intensity ateach wavelength. The wavelength tunable emission spans a wavelengthemission range 210 having a center wavelength 220. In the preferredembodiment of FIG. 1, the tuning region is an adjustable airgap, butother embodiments such as a liquid crystal or semiconductor whoseoptical path can be modified by adjustment of the refractive index arealso possible.

A preferred embodiment of the short-cavity tunable laser of FIG. 1 is avertical cavity laser (VCL), but other embodiments, including but notlimited to short-cavity edge-emitting lasers, could be employed. As iswell-known to those skilled in the art of vertical cavity lasers, theVCL can be fabricated in monolithic one and two-dimensional arrays,which is advantageous for optical systems requiring multiple opticalsources. Modern wafer scale optical fabrication techniques would allowfor the precise location of such an array of laser emitters, as well asoptical components which would then support the manufacturing of opticalinstruments from these arrays.

The short cavity employed in an embodiment results in a largefree-spectral range (FSR), which is inversely related to cavity length.The present embodiment discloses an FSR which in the present inventionis >5% of the center wavelength shown in FIG. 2. As shown in FIG. 3,free spectral range is defined as the distance between transmissionpeaks, or longitudinal modes, in the direction of laser oscillation, ofthe optical cavity defined by the layers of FIG. 1. The maximumcontinuous mode-hop-free single-mode tuning range of the tunable laseris limited by the FSR. Thus, a laser having an FSR that is 5% of thecenter wavelength can be expected to have a maximum tuning range that is5% of the center wavelength. Other considerations, such as the maximumachievable change in optical path length of the tuning region, or theavailable gain bandwidth of the gain region may limit the continuoussingle mode tuning range to less than the FSR, but the FSR represents anupper limit.

In the preferred embodiment, an anti-reflection coating is placedbetween the gain region and the tuning region to suppress reflections inthe device and extend the tuning range. This anti-reflection coating canbe a quarter wavelength of material such as silicon nitride or siliconoxynitride, in the preferred case when the tuning region is air, and thegain region is semiconductor.

FIG. 1 also illustrates that the top mirror 130 can be curved to form ahalf-symmetric cavity as in (Tayebati, P., Wang, P., Vakhshoori, D. etal., “Half-symmetric cavity tunable microelectromechanical VCSEL withsingle spatial mode,” IEEE Photonics Technology Letters, 10(12),1679-1681 (1998)), which includes one curved mirror and one flat mirror.This is important because, although the short cavity and large FSRpromote single longitudinal operation, the curved mirror furtherpromotes single transverse mode operation, which is important forapplications in imaging and spectroscopy. The function of the curvedmirror can also be accomplished by an intra-cavity microlens 160, asshown in FIG. 1. Generally either the microlens 160 or the curved mirror130 can be used, but a combination of both can also be used. Themicrolens can be formed by reflow of a resist or polymer material,followed by pattern transfer of that shape into underlying materials, asis described in (Strzelecka, E. M., Robinson, G. D., Coldren, L. A. etal., “Fabrication of refractive microlenses in semiconductors by maskshape transfer in reactive ion etching,” Microelectronic Engineering,35(1-4), 385-388 (1997)) and known to those who are skilled in the art.Similarly, the curved mirror can be formed by structuring of sacrificiallayer by a reflow and pattern transfer technique, deposition of the topmirror, and removal of the sacrificial layer. The sacrificial layer insuch a process is preferably silicon or germanium, and the patterntransfer of a reflowed resist layer can be accomplished by inductivelycoupled plasma etching using a CF4/oxygen gas mixture. The curvature ofthe resulting surface in the sacrificial layer is a function of theratio of these gases, and can be adjusted by straightforwardoptimization of this ratio.

Achieving single transverse-mode operation of the tunable short cavitylaser in FIG. 1 requires careful control of the curved mirror radius ofcurvature and the combined thickness of gain region and tuning regionbetween the two mirrors. For the case of an airgap tuning region andoperation near 1310 nm using a semiconductor gain region comprised ofInP-based materials, typical dimension are a gain region thickness ofabout 1 micron, airgap thickness of about 1.6 μm, and a mirror radius ofcurvature of around 1 mm. Those skilled in the art of laser fabricationand design can adjust numbers in this range to achieve more specificnumbers for particular designs in particular wavelength regimes. Usingparameters close to these will lead to single longitudinal andtransverse mode suppression of 40-50 dB.

The single longitudinal and transverse mode operation achieved by theshort cavity laser according to an embodiment enables an optical sourcewith very long dynamic coherence length. This coherence length is inexcess of 100 mm under dynamic operation. Coherence length is inverselyrelated to laser line-width. Long coherence length is important inspectroscopic applications requiring the measurement of narrow spectralfeatures. In imaging applications like swept source optical coherencetomography (SS-OCT), long coherence length corresponds to long imagingrange. FIG. 5 shows a detection-limited measurement of coherence lengthin an SS-OCT system, obtained by repetitive sweeping at 60 kHz of atunable laser according to FIG. 1, in which the FSR is about 8-9% of thecenter wavelength, and using the OCT point spread function as ameasurement of coherence length. The absence of substantial amplitudedegradation at 50 mm indicates that the coherent length is greater than100 mm. This measurement method is well-known to those skilled in theart of SS-OCT.

For some applications, it is advantageous to reduce the coherence lengthto eliminate interference from unwanted reflections in an opticalsystem. Coherence length can be adjusted by adding a noise waveform tothe tuning region, or otherwise amplitude or phase modulating thesource. External means could include, for example, a temporal diffuser.

In an embodiment, the frequency response of the optical path length of atuning region to an applied tuning signal has a 6-dB bandwidth thatexceeds about 1 kHz. Normally, this 6-dB bandwidth starts at DC but canstart at some non-zero frequency as well. The 1 kHz bandwidthdistinguishes the present invention from other types of tuningmechanisms employed in the prior art, such as electro-thermal tuning in(Gierl, C., Gruendl, T., Debernardi, P. et al., “Surface micromachinedtunable 1.55 mu m-VCSEL with 102 nm continuous single-mode tuning,”Optics Express, 19(18), 17336-17343 (2011)). In the preferred case wherethe tuning region is an airgap, the airgap can be tuned by a MEMS-basedactuator, which contracts the airgap through electrostatic force.MEMS-based tuning mechanisms have been demonstrated to have a 6-dBbandwidth exceeding 500 kHz, as illustrated in (Jayaraman, V., Cole, G.D., Robertson, M. et al., “High-sweep-rate 1310 nm MEMS-VCSEL with 150nm continuous tuning range,” Electronics Letters, 48(14), 867-9 (2012)).As described below, the bandwidth of a MEMS-actuator can be extendedto >1 MHz. The presence of such a wide bandwidth enables repetitivelyswept operation at a range of frequencies from DC to >1 MHz. It alsoenables non-repetitive wavelength tuning at a variety of scan speeds.The ability to vary the fundamental tuning frequency of the laser withinone laser source makes the instrument appropriate for a broad range ofapplications, each of which have a preferred tuning rate. For example,the measurement of weak spectroscopic signals could require slowscanning speeds, whereas strong spectroscopic signals could be monitoredsuch that dynamic temporal effects could be captured. Many applicationsin SSOCT could also benefit from variable scan frequency, which enablestradeoff of imaging resolution and imaging range with imaging speed.

Although MEMS tuning of an airgap is the preferred embodiment of thepresent invention, an alternative embodiment could tune the airgapthrough a piezo-electric actuator, such as described by (U.S. Pat. No.6,263,002). This configuration is illustrated also in FIG. 11, where thetop mirror 1110 is placed on a piezo-electric actuator and separated byan airgap 1120 from the gain region 1130 and the bottom mirror 1140. Thefirst mirror 1110 is moved relative to the gain region 1130 viapiezo-electric control changing the airgap 1120 tuning region andtherefore, the lasing wavelength. In this structure, the first mirror isattached to a fiber that can deliver pump radiation and collect laserradiation. Piezo tuning can also provide several kHz of bandwidth, whichis generally less than the bandwidth of MEMS tuning, but piezo tuningcan produce larger airgap changes, and can be bi-directional. It is alsopossible to obtain bi-directional tuning in a MEMS device through a3-terminal device. In yet another embodiment, piezo and MEMS tuning canboth be used to provide a combination of a slower, large stroke tuningmechanism, and a faster, small stroke tuning mechanism. These tuningmechanisms can be combined further with other tuning mechanisms such ascarrier injection tuning in semiconductors.

The tuning region can be driven with a variety of waveforms, to generatevarious wavelength trajectories vs. time out of the short-cavity laserfor a variety of applications. For example, the tuning region can bedriven with a repetitive waveform having a fundamental frequency,generating a periodic variation of wavelength vs. time for applicationssuch as swept source optical coherence tomography (SSOCT). The periodicwaveform could be sinusoidal in shape, or an arbitrary waveformconstructed to generate a linearized wavelength sweep vs. time or anywavelength trajectory with time. The response of the tuning region maybe non-linear with respect to the applied waveform. A classic example isMEMS, in which the movement of an electrostatically actuated membranevaries as the square of applied voltage. In this case, creating a linearmovement requires pre-distorting the applied waveform to account for thenon-linear response of the MEMS actuator. The generation of arbitrarywaveforms to linearize MEMS response is well-known to those skilled inthe art of driving MEMS devices, but the principle of linearization canbe applied to other tuning mechanisms as well.

The waveform applied to the tuning region is usually a variation inapplied voltage or current vs. time, depending on the exact nature ofthe tuning region and mechanism of tuning, whether it be changing aphysical path length or changing a refractive index of a semiconductoror liquid crystal, as some representative examples. By way of example,use of a MEMS based tuning element with its very low mass reduces thepower required to sweep the laser wavelength in both a continuous sweepas well as in a non-continuous sweep. The use of a MEMS tuning elementwould require a drive voltage with very little current drawn.

In addition to repetitive wavelength sweeping, the tuning region can bedriven by a non-repetitive waveform, in response to an external trigger,or by any repetitive or non-repetitive arbitrary waveform. Examples ofthis are in transient spectroscopy, where it is advantageous to measurethe transmission, absorption, or reflection spectrum of a materialshortly after an event, such as an explosion, chemical reaction, or abiological event. Non-repetitive scanning would also facilitate newmodes of operation whereby a number of narrow regions of interestseparated by large regions of no interest could be interrogated with thelaser in an optimized manner. One example is a series of slow scansacross narrow spectroscopic features that are separated by large regionswherein the large regions are scanned at high speed. In the preferredcase of MEMS tuning, many new operating modes are made possible by theextremely low mass of the tuning element that allows for rapidacceleration and deceleration of the laser tuning speed.

With respect to scanning in response to an external trigger, theadvantages of a MEMS implementation of the present invention areilluminated by a comparison with the commercially available Thorlabsmodel SL1325-P16 swept source laser (which is not a short-cavity laser).This prior art laser utilizes a resonantly scanned optical gratingmeasuring over 10 mm² as the tuning element, causing slow response timerelative to a low mass MEMS element in the present invention. In anembodiment according to the present invention the very low mass of thetuning element allows greater flexibility in the operating parameters,such as how quickly the laser can respond to an external event, as wellas the wavelength region over which the laser is tuned as a result of anexternal event. This flexibility provides new modes of operation when itis desirable to synchronize the operation of the laser with externalevents.

Having the laser tune across a widely adaptable tuning profile allowsthe performance of the laser to be adjusted to meet the demands of manyapplications. By way of example, in one application it would bedesirable to scan the wavelength of the laser linearly in time if thelaser is being used to measure the wavelength dependence of an opticalelement, in other applications would be desirable to scan the laserlinearly in optical frequency when the laser is being employed to makemeasurements that are best made with samples equally spaced infrequency, such as is the case in Optical Coherence Tomography.

Spectroscopy provides another example of the utility of a highlyflexible tuning profile. In many spectroscopic applications, it isnecessary to measure multiple unequally spaced and variable linewidthlines across a range of wavelengths. Maximizing overall signal to noiseratio requires slower scan speeds in information rich (e.g. many narrowlines) regions of the spectrum and faster scan speeds in regions of thespectrum with less or no information. FIG. 4 shows an example of a watervapor absorption spectrum in the 1330-1365 nm range.

For many applications, such as those described above, the tuning regionof the tunable short-cavity laser according the present region can bedriven open loop—that is, without position or wavelength feedback. Inother applications where wavelength stability is more important,however, feedback control can be employed. This can be advantageous instatic operation, when the wavelength is locked to a particular atomicabsorption line or other atomic reference. Alternately, the wavelengthcan be first locked to an atomic reference and then offset from thisreference using another element to measure the offset distance, such asa Fabry-Perot or Mach-Zehnder interferometer having a known fringespacing. Closed loop control can also be advantageous in dynamicoperation.

FIG. 6 shows a preferred embodiment of closed loop control according tothe present invention. As shown, a portion of the light emitted from thetunable short-cavity laser is split to a wavelength-sensing element 610,which can comprise elements such as a prism, grating, optical filter, oroptical interferometer. In the case of a dispersive element like a prismor a grating, a position-sensing element like a detector array would becombined with the dispersive element to detect diffracted or refractedangle and infer wavelength offset from the desired position and feedthis error signal to the tuning drive waveform 620. If the applicationdoesn't require a specific wavelength but just that a fixed wavelength,or a series of fixed wavelengths be delivered, then the wavelengthdispersing element and the position sensing element could be usedwithout calibration of the dispersing element. In the case of an opticalfilter, the transmission or reflection of the filter as measured by anoptical detector would be used to determine wavelength offset from adesired lock position and feed an error signal back to the tuning regiondrive waveform. Dynamic closed loop operation can be obtained byscanning the error signal, as has been shown in prior art lasers, suchas FIG. 1 of (Roos, P. A., Reibel, R. R., Berg, T. et al.,“Ultrabroadband optical chirp linearization for precision metrologyapplications,” Optics Letters, 34(23), 3692-3694 (2009).) Closed loopcontrol is particularly useful when driving a tuning region at speedswell below a natural resonance, which may create variabilities. Forexample, a MEMS actuator with a 500 kHz resonance driven at 100 kHz maybe prone to variability and may have much more well-behaved tuning withclosed loop control.

FIG. 7 illustrates several details of a preferred implementation of ashort-cavity laser constructed to operate at 1310 nm according to anembodiment of the present invention, and FIGS. 9 and 10 demonstrateseveral additional performance features of the implementation of FIG. 7.FIG. 7 illustrates a semiconductor laser constructed as a verticalcavity surface emitting laser (VCSEL), which is a subset of verticalcavity lasers (VCL). A VCL can emit either downward or upward, andVCSELs emit upward, meaning in the direction opposite the substrate. TheVCSEL of FIG. 7 employs two distributed Bragg reflector (DBR) mirrors710, 720 comprising alternating quarter wave layers of low and highrefractive index material. The DBR is preferred for both mirrors,although a high contrast grating as used by prior art lasers can also beemployed, as described in for example (Chase, C., Rao, Y., Hofmann, W.et al., “1550 nm high contrast grating VCSEL,” Optics Express, 18(15),15461-15466 (2010)).

The bottom mirror 710 of FIG. 7, corresponding to the second mirror 140of FIG. 1, is comprised of alternating quarter wave layers of GaAs andAluminum oxide (AlxOy). This type of mirror is formed by lateraloxidation of an epitaxially grown stack of GaAs/AlAs, as described in(MacDougal, M. H., Dapkus, P. D., Bond, A. E. et al., “Design andfabrication of VCSELs with Al xO y-GaAs DBRs,” IEEE Journal of SelectedTopics in Quantum Electronics, 3(3), 905-915915 (1997)). The GaAs/AlxOymirror has a large reflectivity and wide bandwidth with a small numberof mirror periods. The preferred number of mirror periods for the backmirror, when light is coupled out the top mirror as in FIG. 7, is six orseven periods, creating a theoretical lossless reflectivity of >99.9%.Other implementations of this mirror could use AlGaAs/AlxOy, where thealuminum content of the AlGaAs is less than about 92%, so that it doesnot oxidize appreciably during lateral oxidation of the AlAs to formAlxOy. Use of AlGaAs instead of GaAs for the low index material isadvantageous for increasing the bandgap of the low-index material tomake it non-absorbing at the lasing wavelength or at the pump wavelengthif the laser is optically pumped.

The top suspended mirror 720 of FIG. 7, corresponding to the firstmirror 130 of FIG. 1, is comprised of alternating low and highrefractive index deposited materials, such as for example SiO₂ andTa₂O₅. Other deposited materials could be used as well, including butnot limited to the list consisting of TiO₂, HfO₂, Si, Ag, Al, Au, ZnS,ZnSe, CdF₂, Al₂F₃, and CdS. These materials can be deposited by electronbeam evaporation, ion beam sputtering, plasma-assisted deposition, orother means well-known to those skilled in the art. For the example, incase of a 10 period SiO2/Ta₂O₅ period mirror having refractive indicesof 1.46/2.07 respectively, centered in a range of about 700 nm to about1600 nm, the theoretical lossless reflectivity exceeds 99.5% over arange of at least 10% of the center wavelength, as can be calculated bythose skilled in the art of mirror design.

The implementation of FIG. 7 uses MEMS actuation to control thethickness of an airgap tuning region to control the output wavelength ofthe device in the range of 1310 nm. Application of a voltage between theactuator contacts 730, 740 shown contracts the airgap and tunes thelaser to shorter wavelengths. The MEMS structure shown consists of arigid supporting structure 750 and a suspended deformable dielectricmembrane 760, on which is the suspended top mirror 720. The top of thedielectric membrane 760 is metallized to enable electrostatic force tobe applied by the actuator contacts 730, 740. The membrane itself istransparent, runs underneath and is integral with the suspended mirror,and contributes constructively to the reflectivity of the suspendedmirror. Ideally the membrane thickness is an odd number of quarterwavelengths at the center wavelength of the emitted tuned radiation. Formany wavelengths of interest, such as in the 600-2500 nm range, theideal thickness is about ¾ wavelength.

In the preferred embodiment, the dielectric membrane is silicon nitride,which is a robust material, which can be stress-engineered to create thedesired frequency response. Ideally the silicon nitride has a tensilestress in the range or about 100 to about 1000 MPa. This range of stressleads to a lowest order resonant frequency of the MEMS actuator,described below, that is substantially increased by the stress. Althoughtensile stress is preferred, compressive can also be employed, though itis less preferred, since it leads to bowing of the membrane after MEMSrelease. Other authors have made advantageous use of this bowing tocreate a half-symmetric cavity, as described for example in (Matsui, Y.,Vakhshoori, D., Peidong, W. et al., “Complete polarization mode controlof long-wavelength tunable vertical-cavity surface-emitting lasers over65-nm tuning, up to 14-mW output power,” IEEE Journal of QuantumElectronics, 39(9), 1037-10481048 (2003)).

The representative preferred embodiment is shown in FIG. 7 is configuredto operate at 1310 nm. This configuration therefore uses an InP-basedmulti-quantum well (MQW) region comprising at least one quantum well inthe gain region. Since the bottom fully oxidized GaAs/Al_(x)O_(y) mirroris grown on GaAs instead of InP, the InP-based MQW region must be joinedto the GaAs-based fully oxidized mirror through a wafer bonding process,as described in fixed wavelength 1310 nm VCSELs such as in (Jayaraman,V., Mehta, M., Jackson, A. W. et al., “High-power 1320-nm wafer-bondedVCSELs with tunnel junctions,” IEEE Photonics Technology Letters,15(11), 1495-14971497 (2003)). The multi-quantum well region ispreferably comprised of multiple compressively strained AlInGaAs quantumwells, with strain in a range of 1-1.5%. In another embodiment, it ispossible to use a GaInNAs quantum well that can emit around 1310 nm andcan be grown on GaAs, eliminating the need for wafer bonding. TheAlInGaAs quantum well is however higher gain and more wavelengthflexible, and is therefore preferred.

FIGS. 25 and 26 illustrate the major steps of a fabrication sequenceused to fabricate the preferred implementation of the 1310 nm tunableshort cavity laser in FIG. 7. Processing of devices in a wavelengthrange of 650-2300 nm can proceed in a similar fashion, with the exceptthat GaAs-based devices do not require the first wafer bonding stepshown in FIG. 25, since mirror and gain region can be epitaxially grownin one step. As shown in FIG. 25, at 1310 nm, or at any wavelengthemploying an active region grown on InP, the first step 2510 involveswafer bonding of the MQW region epitaxially grown on an InP substrate toa GaAs/AlAs mirror structure epitaxially grown on a GaAs substrate. Thisprocess can be accomplished through the application of pressure andabout 570 C temperature for about 15 minutes, as has been described ingreater detail by prior art researchers in (Black, A., Hawkins, A. R.,Margalit, N. M. et al., “Wafer fusion: Materials issues and deviceresults,” IEEE Journal of Selected Topics in Quantum Electronics, 3(3),943-951 (1997)). The MQW and the mirror structure are joined at awafer-bonded interface. After bonding, the InP substrate is removed in asecond step 2520 using an HCL-based etch stopping on an InGaAs stop etchlayer. A sulfuric acid based etch then removes the stop-etch layer.

In a third series of steps 2530, the bottom MEMS contact, which ispreferably terminated with titanium to promote adhesion of germanium ina subsequent step, and anti-reflection coating are deposited andpatterned, and holes are etched for oxidation of the GaAs/AlAs mirrorstructure. Oxidation converts the AlAs to Al_(x)O_(y) to create a highlyreflecting mirror with six or seven periods. After mirror oxidation, agermanium sacrificial layer is deposited in a fourth step 2540, and thegermanium is structured to have a curved surface in the region of lightoscillation. This curved surface is created by a photoresist reflow andpattern transfer process, using an oxygen-rich CF₄/O₂ inductivelycoupled plasma etch process. FIG. 26 illustrates how in a 5^(th) seriesof steps 2550, the silicon nitride membrane layer, top actuator contact,and top suspended mirror are deposited and patterned on the germaniumsacrificial layer. The top contact layer is preferably aluminum.

In a 6^(th) series of steps 2560, the germanium sacrificial layer isreleased using Xenon Diflouride (XeF₂) gas to create a suspendedstructure with a rigid germanium support structure at the edges. Priorto the germanium release, wire bond pads, shown in FIG. 7 connectingwith the top and bottom actuator metal, are deposited to completeprocessing before release. Processing after release is generally notadvisable, as it can lead to collapse of the actuator. In many cases, itis preferable to dice and wire bond devices to a submount for packaging,prior to releasing the germanium membrane.

The design of the gain region in the preferred embodiment of FIG. 7 isimportant for device performance. In the case where the structure isoptically pumped, the quantum wells can be aligned with peaks of anoptical standing wave to enhance gain through the well-known periodicgain effect, described in the prior art by (Corzine, S. W., Geels, R.S., Scott, J. W. et al., “DESIGN OF FABRY-PEROT SURFACE-EMITTING LASERSWITH A PERIODIC GAIN STRUCTURE,” IEEE Journal of Quantum Electronics,25(6), 1513-1524 (1989)). One further advantage of periodic gain is thatthe wide spacing between quantum wells prevents strain accumulation andreduces the need for strain compensation. The ideal pump wavelength forthe 1310 nm tunable VCSEL shown is in a range of about 850-1050 nm. Inan optically pumped structure, three quantum wells can be placed onthree separated standing wave peaks, and the region between them can bemade of AlInGaAs substantially lattice-matched to InP, and of acomposition that absorbs incoming pump radiation. Thus the gain regionis separated from the absorbing regions, and photo-generated carriers inthe absorbing regions diffuse into gain region. Alternately, the FSR ofthe structure can be increased by placing three quantum wells at asingle standing wave peak. In this case, strain compensation of thecompressively strained AlInGaAs wells with tensilely strained AlInGaAsmay be required. This creates a thinner absorbing region, sinceabsorption may occur only in the quantum wells. Such a structure willrequire more pump power in an optically pumped device, but will providewider tuning range. One implementation of FIG. 7 using three quantumwells on a single standing wave peak enabled a structure with 161 nm FSRnear 1310 nm, representing 12.3% of the center wavelength, as shown inthe tuning results of FIG. 9. Continuous single-mode tuning range withthis device was 150 nm, as also shown in FIG. 9 and discussed morebelow. FIG. 9 shows the static and dynamic tuning response of anultra-widely tunable MEMS-VCSELs. The long-wavelength spectrum 910 at1372 nm exhibits a completing mode at 1211 nm, illustrating the 161 nmFSR of the cavity. The curve 920 represents the time-averaged spectrumunder sinusoidal sweeping at 500 kHz. Both the static and dynamicresponse demonstrate continuous single-transverse and longitudinal modelasing operation over a 150 nm span. FSR in the range of 140-170 nm for1310 nm devices provides device tuning that is exceptionally well suitedfor swept source optical coherence tomography systems. A large FSR isdesirable not only as a means to increase the tuning range of the laserbut also as a means to reduce the duty factor of the laser such thatadditional tuning profiles from other tunable short-cavity lasers can beadded as described later. For example, if the gain bandwidth of thelaser is restricted to <50% of the FSR, and the entire FSR is swept,then the laser automatically turns off for more than half the sweep,leaving room to interleave a sweep from another laser, or a time-delayedcopy of the sweep, as described in for example (Klein, T., Wieser, W.,Eigenwillig, C. M. et al., “Megahertz OCT for ultrawide-field retinalimaging with a 1050 nm Fourier domain mode locked laser,” OpticsExpress, 19(4), 3044-30623062 (2011)).

When it is desirable to maintain lasing over a very wide tuning range(>10% of center wavelength), it is advantageous to broaden the gain ofthe quantum wells by including a second confined quantum state in thewell by using wider quantum wells as described in (U.S. Pat. No.7,671,997). We note that the wide FSR structure producing the results ofFIG. 9 employed can be characterized by the number of maxima in theoptical standing wave formed between the mirrors during lasingoperation. The results of FIG. 9 were produced by a structure with fivestanding wave maxima in the cavity between the bottom mirror and thesuspended mirror. Further reduction of cavity thickness to below fivestanding wave maxima can lead to larger FSR approaching 200 nm for a1310 nm device. FIG. 8 shows the variation of refractive index vs. depthalong the axis of laser oscillation for an example 1310 nm design withfour standing wave maxima between the two mirrors. The periodicstructure at the left of FIG. 8 represents the fully oxidized mirror andthe periodic structure at the right of FIG. 8 represents the suspendeddielectric mirror including the thicker first layer which is the siliconnitride membrane. The MQW gain region and airgap tuning region betweenthe mirrors are also indicated in FIG. 8.

The features described in the preceding paragraph apply not only toVCSELs in the 1310 nm range but similar principles can be applied acrossthe 1150-2500 nm wavelength range, which can use an InP-based gainregion in conjunction with a GaAs-based mirror region. The 1200-1400 nmrange is particularly important for many swept source optical coherencetomography applications, such as endoscopic applications, vascularimaging, and cancer imaging. The 1800-2500 nm range is important for gasspectroscopy. This latter range preferably uses compressively strainedInGaAs quantum wells on Indium Phosphide substrates.

FIG. 23 illustrates another MEMS-tunable VCSEL like that of FIG. 7 butinstead configured to operate in a wavelength range around 1060 nm. Likethe 1310 nm VCSEL, this device employs a fully oxidizedAlGaAs/Al_(x)O_(y) mirror 2310 as the bottom mirror. The aluminumcontent in the AlGaAs layers of this bottom mirror is preferably >10%,to prevent absorption of the pump beam in the optically pumpedstructure, which ideally has a pump wavelength around 850 nm. In thiscase, no wafer bonding is required, since the compressively strainedInGaAs quantum wells in the gain region can be epitaxially grown on thesame GaAs substrate as the fully oxidized mirror. A non-wafer-bondedstructure like FIG. 23 can be configured with other quantum wellcompositions to access a range of wavelengths in a range from about 600nm to about 1150 nm. Besides InGaAs, these include but are not limitedto AlInGaP, AlInGaAs, InGaAsP, InGaP, AlGaAs, and GaAs. GaAs quantumwells would be used in about the 800-870 nm range, AlGaAs wells in aboutthe 730-800 nm range, AlInGaP and InGaP in about the 600-730 nm range,and InGaAsP or AlInGaAs as alternative materials in about the 800-900 nmrange. The wavelength range of 700-1100 nm is of particular interest inSSOCT ophthalmic imaging and also oxygen sensing, and the range of about990-1110 nm is of greatest interest for ophthalmology.

As in the case of the 1310 nm structures, periodic gain can be employedin the structure of FIG. 23. In the 990-1110 nm range, it isadvantageous to use a periodic gain structure with three InGaAs quantumwells 2320 at three standing wave peaks in the cavity, separated by GaAsbarriers which absorb the pump radiation and generate electrons andholes which diffuse into the quantum wells. Typical quantum well widthsare 6-12 nm and typical Indium percentage is about 20%. Quantum wellwidths greater than about 8 nm lead to a second confined quantum stateand broadened gain. A structure using this approach generated the tuningresults shown in FIG. 24, illustrating a tuning range of 100 nm around1060 nm. The FSR of this structure is around 100 nm or about 9.4% of thecenter wavelength. FSR can be increased to >10% as in the 1310 nmstructure by placing all quantum wells on a single standing wave peak orby placing four quantum wells on two standing wave peaks. In theselatter cases, strain compensation of the InGaAs with tensile-strainedGaAsP, as described in the prior art on fixed wavelength VCSELs(Hatakeyama, H., Anan, T., Akagawa, T. et al., “Highly ReliableHigh-Speed 1.1-mu m-Range VCSELs With InGaAs/GaAsP-MQWs,” IEEE Journalof Quantum Electronics, 46(6), 890-897 (2010)) can be employed.

Both the structure of FIG. 7 and that of FIG. 24 achieve a tuning rangethat is >90% of the FSR of the device, as shown in the associatedresults of FIGS. 9 and 24 respectively. Also shown in FIGS. 9 and 24 isa suppression of transverse modes, manifesting themselves as a shoulder1-3 nm away from the main peak, which is generally >40 dB below the mainpeak. In an optically pumped structure employing a single transversemode pump beam, the transverse mode suppression can be increased to >45dB across the tuning range if the pump beam is well-aligned along theoptical axis of the half-symmetric cavity of FIG. 1.

The specific implementation of the embodiments in FIGS. 7, 9, 23, 24employs materials and wavelength ranges associated with GaAs and InPsubstrates. Other materials could be used to implement some embodimentsof the present invention. For example, tunable emission in about the2000-2500 nm mid-infrared range could be obtained using materials onGaSb substrates, as prior art researchers have done with fixedwavelength VCSELs in (Kashani-Shirazi, K., Bachmann, A., Boehm, G. etal., “MBE growth of active regions for electrically pumped, cw-operatingGaSb-based VCSELs,” Journal of Crystal Growth, 311(7), 1908-1911(2009)). Alternately, a tunable short-cavity laser according to anembodiment of the present invention operating in the 400-550 nm rangecould be realized using materials grown on GaN substrates as describedby researchers making fixed wavelength VCSELs (Higuchi, Y., Omae, K.,Matsumura, H. et al., “Room-Temperature CW Lasing of a GaN-BasedVertical-Cavity Surface-Emitting Laser by Current Injection,” AppliedPhysics Express, 1(12), (2008)). Implementation of embodiments of thepresent invention in the visible range of 400-700 nm range hasapplication in optical metrology tools and biological and medicalspectroscopy.

One preferred embodiment for all the wavelength ranges indicated aboveis an optically pumped embodiment in which an optical pump suppliesenergy for lasing, as in many examples already discussed. For operationin the 550-700 nm range, the optical pump wavelength is preferably in arange of about 400 nm to about 600 nm. For operation in the 700-1100 nmrange, the preferred pump wavelength is in a range of about 600-1000 nm.For operation in the 1200-1400 nm range, the preferred pump wavelengthis in a range of about 700-1200 nm. For operation in the 1800-2500 nmrange, the preferred pump wavelength is in a range of about 1000-2000nm. We note that it is often advantageous to pump through the topmirror, as indicated in the 1050 nm MEMS-VCSEL of FIG. 24. Side pumpingaround the mirror is also possible, but pumping through the top mirrorleads to a more compact package. In this case the top mirror needs tohave minimal reflectivity at the pump wavelength. FIG. 14 illustrates anexample top mirror designed reflectivity for a tunable short-cavitylaser configured to emit in the range of 1200-1400 nm, with an opticalpump at 1050 nm. As shown in FIG. 14, the top mirror can be made to haveminimal reflectivity 1410 at the pump wavelength at 1050 nm, whilehaving high reflectivity 1420 at the desired 1200-1400 nm emissionwavelength range.

Although the above has been primarily described with respect tooptically pumped devices, transition from optical pumping to electricalpumping can use well-known processing methods for vertical cavitylasers. An example electrically pumped structure according to anembodiment of the present invention is illustrated by FIG. 27, which isa MEMS-tunable VCSEL with GaAs-based MQW gain region 2710 and a fullyoxidized mirror, as in the 1060 nm example of FIG. 24. As shown in FIG.27, the bottom MEMS contact 2740 also functions as the top laser diodecontact. In the optically pumped structure, the confinement of opticalcarriers is accomplished by the limited lateral extent of the opticalpump beam, while in an electrically pumped structure a current aperture2720 must be provided. This aperture 2720 is usually provided by anotherpartially oxidized layer above the fully oxidized mirror, as shown inFIG. 27. The current aperture could also be provided by a patterned andburied tunnel junction, as has been employed by other researchers. Inboth cases, care must be taken to engineer the spreading resistance toavoid current crowding, as has been described by prior art researchersin fixed wavelength VCSELs (Scott, J. W., Geels, R. S., Corzine, S. W.et al., “MODELING TEMPERATURE EFFECTS AND SPATIAL HOLE-BURNING TOOPTIMIZE VERTICAL-CAVITY SURFACE-EMITTING LASER PERFORMANCE,” IEEEJournal of Quantum Electronics, 29(5), 1295-1308 (1993)).

As shown in FIG. 27, the combination of implant passivation 2730 andoxide current aperturing 2720 enables electrical pumping of thestructure. Current conduction proceeds from the middle MEMS contact 2740through the current aperture 2720, and around the fully oxidized regionof the bottom mirror to a backside substrate contact 2750. Carrierrecombination in the MQW region, which is preferably comprised of threestrain-compensated InGaAs/GaAsP quantum wells, produces gain for lasing.

For many applications of interest, it is desirable to control thespectral shape of the output power spectrum emerging from the tunableshort cavity laser. This output power shaping can be accomplished in avariety of ways. One method is by controlling the shape of the topmirror reflectivity spectrum. Generally, regions of lower reflectivityallow more light out of the optical cavity, while regions of higherreflectivity allow less light out of the optical cavity. Thus, one candefine a target spectral shape or power variation across the wavelengthrange, and adjust a shape of the mirror reflectivity achieve thisspectrum. A target power variation might be a Gaussian shape. FIG. 22illustrates examples of several spectral shapes that have been achievedin the MEMS-VCSEL implementation of FIG. 7, by adjusting thereflectivity spectrum of the suspended top mirror. These spectra rangefrom power peaked at both edges, power peaked at the blue edge, andpower peaked at the red edge. Additional spectral shapes can be achievedby the same method.

Another way of changing the spectral shape is to control the pump energyinto the gain region dynamically during wavelength tuning. In the caseof an optically pumped device, this can be controlling the pump energyinto the device, and in the case of an electrically pumped device thedrive current would be controlled. Shaping of the pump energy can alsoimprove thermal management of the device.

For the particular embodiment that uses a MEMS actuator, further detailsof the MEMS actuator design can be implemented to enhance the deviceperformance. As mentioned above, the deformable dielectric membrane ispreferably made of silicon nitride, and a tensile stress of 100-1000 MPais preferred to give a substantially increased resonant frequencyrelative to a no-stress design, and to minimize bowing of the membraneupon release. By resonant frequency, we are referring to the lowestorder mechanical mode of the device, which corresponds to the desired“piston” motion of the actuator. This is an important parameter of thedevice performance. One preferred actuator geometry is a central platewith supporting arms, as shown in FIG. 7, FIG. 12, and FIG. 23.Important parameters of this particular geometry are the actuatordiameter, central plate diameter, arm width, and suspended mirrordiameter, as shown in FIG. 23. Using an actuator diameter of about 220μm, between four and eight supporting arms, an actuator arm width ofabout 16 μm, a suspended mirror diameter of about 34 μm, a suspendedmirror comprised of about 11 periods of SiO₂/Ta₂O₅ centered at 1310 nm,a central plate diameter varying from about 50 μm to about 110 μm, and a¾ wavelength silicon nitride membrane with stress in a range of about200 MPa to about 450 MPa, it is possible to obtain a variety offrequency responses represented by the sampling of FIG. 10. FIG. 12shows pictures of a sampling of actuator geometries resulting in thefrequency responses of FIG. 10. In FIG. 12, top-view pictures of severalMEMS tunable VCSEL structures having four or eight supporting struts1210. FIG. 10 shows the tuning of a MEMS-VCSEL wavelength as a functionof drive frequency applied to the MEMS-actuated airgap tuning mechanism.As shown, the resonant frequencies are in a range of about 200 kHz toabout 500 kHz, and the 6 dB bandwidths of the fastest devices areapproaching 1 MHz.

Also shown is a variation in the damping of the actuator, manifested byvarying amounts of peaking at resonance. The damping is primarily causedby squeeze-film damping, which represents interaction with viscous air.As the actuator area is increased or the airgap is reduced, thesqueeze-film damping goes up, flattening the frequency response. A flatwide frequency response is desirable for variable speed drive, and forlinearization of drive through multiple harmonics. Though dampingthrough squeezed film effects is demonstrated in FIG. 10 in a MEMSdevice, similar effects can be seen in other airgap tuned devices suchas piezo-driven devices. In general, it is possible to control thedamping of the MEMS actuator through a variety of methods, includingchanging the actuator area or shape to change interaction with viscousair, changing the background gas composition or gas pressure, whichfurther changes the contribution of squeeze-film damping, changing theairgap thickness, and changing the size of holes or perforations in theactuator to change the regime of fluid flow through the holes from aturbulent to a non-turbulent regime. Additionally, annealing theactuator can change the stress of various materials in the actuator,which will have an effect on damping.

The frequency responses represented by FIG. 10 are representative andnot limiting. The resonance frequency can be increased by stiffening themembrane through increased tensile stress, increased thickness (forexample 5/4 wavelength), reduced suspended mirror diameter andthickness, or shortened arms, such that 6-dB bandwidths in excess of 2MHz can be achieved, as can be calculated by those skilled in the art offinite element modeling. Similarly, resonant frequency can be decreasedwell below 100 kHz by changing the same parameters in the oppositedirection. We also note that other geometries are possible, such as aspiral arm geometry, which reduces resonant frequency, or a perforatedmembrane without clearly delineated supporting struts. Referring to FIG.12, if the diameter of the central plate 1220 is expanded to the outerring 1230 actuator diameter, and perforations are added, we achieve aperforated membrane without clearly delineated supporting struts.

The silicon nitride membrane discussed above is highly insulating, andmay therefore be prone to charging and electrostatic drift. Introducinga small amount of electrical conductivity in the membrane can reduce thepropensity to charging. For silicon nitride, this electricalconductivity can be introduced by using a non-stoichiometricsilicon-rich film, or by doping the silicon nitride film with silicon.

The tuning results presented in FIGS. 9 and 24 indicate the voltagesrequired to tune the device, noted alongside the spectra. These voltagesrange up to about 65 V for full tuning over on FSR, corresponding to amembrane deflection of about half the center wavelength or about 650 nmfor 1310 nm devices and 525 nm for 1050 nm devices. These voltages areassociated with the MEMS actuator dimensions and silicon nitride stresslevels indicated above, and with reference to FIGS. 10 and 12, alongwith a nominal zero-voltage airgap in the range of about 1.6 μm.

We also note that faster tuning mechanisms than mechanical contractionor expansion of an airgap can be employed such as carrier injectiontuning in semiconductors, which can be in the GHz range. This mechanism,however, is typically limited to about a 1% change in optical pathlength, so is not suitable for large tuning ranges.

A number of additional structural and performance features of anembodiment of the present invention can be understood with furtherreference to FIGS. 1 and 2. For many applications, it is desirable tohave the intensity vs. wavelength profile, shown in FIG. 2, to be freeof periodic variation. The present disclosure describes a short-cavitytunable laser with a ripple that is less than about 1% of an averagepower. The term “ripple” is commonly used to describe these variations.Depending on the spectral period of this ripple, and depending on theapplication, it may have varying degrees of adverse effect. For example,in a swept source OCT (SSOCT) system, ripple of a particular spectralperiod having an amplitude of 1% or more relative to an average powercan manifest itself as a spurious reflector at an apparent distance inan SSOCT image. Ripple is typically caused by spurious reflectionsoutside the laser cavity. These reflections can come from couplinglenses or other optical elements in the optical system, or they can comefrom substrate reflections in a vertical cavity laser. For example, inthe laser of FIG. 7, reflections coming from below the second mirror,such as from the bottom of the GaAs substrate 770 on which this deviceis disposed, can cause ripple. The substrate reflection amplitude can besuppressed by various means, including but not limited to increasing thereflectivity of the second mirror, introducing loss through dopants inthe substrate, increasing substrate thickness, or roughening thebackside of the substrate to increase scattering. An optimal grit forsubstrate roughening to increase scattering is a grit size >30 μm in therange of 900-1400 nm tunable emission. In addition, use of a fullyoxidized bottom mirror having 7 or more periods, which has a theoreticallossless reflectivity of >99.5%, can suppress ripple to <1% levels.

Another important performance feature of an embodiment of the presentinvention is operation in a fixed polarization state throughout a tuningrange of the wavelength swept emission. Semiconductor lasers in whichlasing emission occurs perpendicular to the plane of a strained quantumwell, such as vertical cavity lasers, have no natural preferredpolarization unless some non-symmetry is introduced into the cavity.Operation in a single polarization state is important if operating withany polarization-sensitive components in the optical system, such aspolarization-selective optical amplifiers. Such systems may also employthe polarization stable device according to an embodiment of the presentinvention in combination with polarization maintaining fiber.Polarization switching over the emission wavelength range can causepower dropouts or image artifacts in an SS-OCT system, and compromisedynamic coherence length. Having a well-defined polarization state wouldalso allow a laser system to be constructed that requires alternatingpolarization states.

Operation in a single polarization state throughout a tuning range ofthe device can be accomplished in a variety of ways. One way is tointroduce one or more nanowires integral with the optical cavity of thedevice. With respect to FIG. 7, this nanowire can be disposed on top ofthe MQW gain region 780 adjacent the tunable airgap, in the center ofthe optical path. Alternatively it could be placed on top of thesuspended mirror. A nanowire is an element which can causepolarization-dependent scattering or absorption of light. Typicaldimensions might be 50 nm wide, several microns long, and 10 nm thick.The nanowire might be constructed of metal or may simply be a refractiveindex perturbation. Typically light polarized along the long directionof the nanowire interacts with a different strength than that polarizedperpendicular to the nanowire. Since laser cavities require smallamounts of loss anisotropy for mode selection, a single intra-cavitynanowire is sufficient to suppress one polarization while maintaininglow loss in another polarization. The loss in different polarizationsfor a nanowire can be calculated by known means to those skilled in theart as described for example in (Wang, J. J., Zhang, W., Deng, X. G. etal., “High-performance nanowire-grid polarizers,” Optics Letters, 30(2),195-197 (2005)). Having a grid of nanowires creates greater lossanisotropy, but also increases loss for the preferred polarization. Soin for example a VCSEL cavity, excess loss introduced in the preferredpolarization should be <0.1%. This suggests one or a very small numberof nanowires. In the case of a VCSEL or VCL, ideally the nanowire shouldbe aligned with the crystal axes of the semiconductor on which the VCSELis disposed. This typically means the [110] direction or perpendicularto the [110] direction for wafers grown on (100) or near (100)orientation. The reason for this is that a weak polarization selectioneffect exists to align the VCL polarization along one of the crystalaxes, and any further polarization control method should strive to addto rather than compete with this effect.

Other means of polarization control include introduction of anisotropicstress, as in (Matsui, Y., Vakhshoori, D., Peidong, W. et al., “Completepolarization mode control of long-wavelength tunable vertical-cavitysurface-emitting lasers over 65-nm tuning, up to 14-mW output power,”IEEE Journal of Quantum Electronics, 39(9), 1037-10481048 (2003)),lateral current injection as described in fixed wavelength VCSELs(Zheng, Y., Lin, C.-H., and Coldren, L. A., “Control of PolarizationPhase Offset in Low Threshold Polarization Switching VCSELs,” IEEEPhotonics Technology Letters, 23(5), 305-307 (2011)), or use of anon-circularly symmetric oxidation process to create the fully oxidizedmirror of FIG. 7, as described with respect to FIG. 13. As shown in FIG.13, oxidation 1310 proceeds outward from two etched holes 1320, andoxidation fronts meet along a line shown by the dashed line 1330 in thefigure. Along this dashed line is a 5 nm dip, which forms a refractiveindex nanowire, which can select the VCSEL polarization. The refractiveindex nanowire of FIG. 13 will be aligned with the crystal axes as longas the holes are aligned with the crystal axes.

Further enhancement of polarization control can be obtained inwafer-bonded devices by ensuring that crystal axes of the bonded wafersare aligned during the bonding process. Since one crystal axis may beslightly preferred over another, aligning crystal axes during bondingleads to multiplication of this effect, rather than cancellation of theeffects by crossing the alignments.

The tunable short-cavity laser described here can be combined in arrayform to generate an aggregate tunable laser source with enhanced opticalproperties. In the particular implementation where the laser is aMEMS-tunable vertical cavity laser, the array can be fabricated inmonolithic form. One example of such combination of particular utilityin SS-OCT is described with the aid of FIG. 28. As shown in FIG. 28A, afirst tunable short cavity laser TCSL 1 and a second tunableshort-cavity laser TCSL 2 are multiplexed on to a common optical path,using a beam splitter, fiber coupler or other know combining element2810. Each TCSL is driven to have a bidirectional tuning over its tuningrange, as shown by the solid wavelength trajectory 2820 in FIG. 28C forTCSL 1 and the dashed trajectory 2830 in FIG. 28C for TCSL 2. Each laseris repetitively scanned at a repetition period T, but the scan of TCSL 2is time-delayed relative to that of TCSL 1 by half the repetitionperiod. In addition, the pump energy 2840, 2850 (either electrical oroptical pump) for each of the two TCSLs is turned off during thebackward wavelength scan such that only the forward or front of half ofthe wavelength scan, when pump energy is non-zero, emits laserradiation. In some instances, if the FSR is much larger than the gainbandwidth of the supporting material, scanning the tuning element beyondthe material gain bandwidth will automatically shut off the laserwithout having to turn off the pump energy.

The wavelength trajectory of the multiplexed output is shown in FIG.28D, comprising components from both TCSL 1 (solid) 2860 and TCSL 2(dashed) 2870, and illustrating unidirectional scanning at a newrepetition period T/2 which is half the original period T of each TCSL.In this way, the sweep rate has been multiplied by a factor of two. Thesame principle could be applied to N lasers and multiplication of thesweep rate by a factor of N. The principle of interleaving TCSLs canalso be used for more than multiplying sweep rate, but also formultiplying tuning range, interleaving different tuning ranges, tuningspeeds, or tuning trajectories, or a for a variety of other purposesevident to those skilled in the art of SSOCT, spectroscopy,communications, or optical detection.

The tunable short-cavity laser described thus far can be combined withan optical amplifier to create an amplified tunable source withincreased output power and other advantageous properties for imaging.The amplifier can be a semiconductor amplifier, a fiber amplifier suchas a praseodymium-doped fiber amplifier for operation in a window around1300 nm, an Ytterbium-doped amplifier for operation in a window around1050 nm, a Fluoride-doped extended bandwidth fiber amplifier near 1050nm, or any kind of optical amplifier. The use of an amplifier can alsoenable the interleaving scheme above, wherein a high extinction ratiooptical amplifier can be used to turn on one source at the appropriatetime, instead of turning off the pump energy to that source.

One basic configuration is illustrated in FIG. 15, in which a tunableshort cavity laser 1510 according an embodiment of the present inventionemits an input tunable radiation 1520 directed to an input side of theoptical amplifier 1530. This input tunable radiation has an inputaverage power, input power spectrum, input wavelength range, and inputcenter wavelength. The amplifier amplifies the input tunable radiationto generation an output tunable radiation having an output averagepower, output center wavelength, output wavelength range, and outputpower spectrum.

In the preferred embodiment, the amplifier is operated in a saturatedregime, as is well-known to those skilled in the art of opticalamplifiers. The saturated regime can suppress noise fluctuations presentin the input tunable radiation, and can also provide advantageousspectral shaping in which a full-width at half-maximum (FWHM) of theoutput tunable radiation can exceed a FWHM of the output tunableradiation. An example of this is shown in FIG. 21, in which theamplified tunable spectrum 2110 has a wider FWHM than the input tunableradiation 2120 from the tunable short cavity laser.

In the preferred embodiment the optical amplifier is a semiconductorquantum well amplifier, which can provide low noise, widegain-bandwidth, and high gain. Semiconductor quantum well amplifiers canalso provide very high extinction ratio >40 dB, which can be used as aswitch to gate devices on and off as described above. The quantum wellis preferably configured to have two confined quantum states to supporta wider gain bandwidth. FIG. 16 illustrates amplified spontaneousemission from a dual quantum state semiconductor optical amplifier at1310 nm, comprising three AlInGaAs compressively strained quantum wells,illustrating a hump 1610 at the shorter end of the spectrumcorresponding to second quantum state widening of the spectrum. The 3-dBspectral width of this amplified spontaneous emission (ASE) is 110 nm,suggesting a 3 dB small signal gain bandwidth of similar value.

The semiconductor optical amplifier can be configured to be polarizationsensitive, by using all compressively strained or tensile-strainedquantum wells, or polarization insensitive by using both types of strainin a single structure to provide gain at all polarizations.

In the preferred configuration, the center wavelength of the inputtunable radiation is at a longer wavelength than a center wavelength ofamplified spontaneous emission (ASE) emitted by the amplifier. Theamplifier ASE is typically blue-shifted relative to the amplifier gainspectrum, so this configuration brings the spectrum of input tunableradiation into more optimal alignment with the amplifier gain spectrum.In general, varying the alignment of the amplifier ASE relative to theinput power spectrum can provide advantageous spectral shaping.

The basic configuration of FIG. 15 can be augmented with various formsof filtering to create a lower noise amplified swept source. Many sweptsource laser application in metrology, spectroscopy, and biophotonicswould benefit from the suppression of broadband ASE, and an improvementin side mode suppression. The addition of an additional tunable spectralfilter to the system, either internal to the laser cavity, between thelaser and amplifier, or at the output of the system is one means ofproviding improved performance in this regard. In one preferredembodiment, the amplifier shown in FIG. 15 can be a tunable resonantamplifier, such as a vertical cavity amplifier described by (Cole, G.D., Bjorlin, E. S., Chen, Q. et al., “MEMS-tunable vertical-cavitySOAs,” IEEE Journal of Quantum Electronics, 41(3), 390-407 (2005)),which only amplifies at a narrow band of wavelengths, and issynchronously tuned with the input tunable radiation of the tunableshort cavity laser, such that the passband of the amplifier is alwaysmatched to the input tunable radiation wavelength.

A number of other preferred configurations are illustrated by FIGS.17-20. In FIG. 17, a synchronously tuned optical filter 1710, whosepassband is aligned at all times with the wavelength of the inputtunable radiation, is placed after the broadband optical amplifier 1720to reduce residual ASE noise and improve a signal to noise ratio of theamplified tunable radiation. In FIG. 18, the same synchronously tunedoptical filter 1810 is placed between the tunable short cavity laser1830 and the optical amplifier 1820, to improved a side-mode suppressionof the input tunable radiation prior to amplification.

Another configuration is illustrated in FIG. 19, where two amplificationstages 1910, 1920 are used. These can be implement as two separateamplifiers, or as a single waveguide amplifier with split amplifiercontacts. The use of two amplification stages 1910, 1920 providesfurther flexibility in spectral shaping. For example, the gain spectrumof the two amplifiers can be shifted relative to each other, either bybiasing identical epitaxial structures differently, or by usingdifferent epitaxial structures in the two amplifiers. The use of twoamplification stages can also create higher gain and greater outputpower.

FIG. 20 illustrates yet another two-stage amplifier configuration inwhich a synchronously tuned optical amplifier 2030 is placed between thetwo amplifier stages 2010, 2020. This will serve to provide an improvedsignal to noise ratio of the output tunable radiation.

In most cases of practical interest, in optical systems such as SSOCTand optical spectroscopy, the preferred range of input average powers isabout 0.05-2 mW, resulting in a preferred range of output average powersof about 10-120 mW. The exact numbers depend on the gain and saturationpower of the amplifier, but this range generally produces amplifiedtunable radiation with good signal to noise ratio for optical systems.

The basic configuration of the tunable short-cavity laser in combinationwith an amplifier can be realized with semiconductor optical amplifiersemploying a variety of materials appropriate for a variety of wavelengthranges. For example, the amplifier can operate in the 1200-1400 nm rangeappropriate for SSOCT and water vapor spectroscopy. In this range, useof an AlInGaAs or InGaAsP quantum well on InP produces the requiredgain. Alternately, the amplifier can operate in about the 800-1100 nmrange appropriate for ophthalmic SSOCT, employing at least onecompressively strained InGaAs quantum well.

The tunable short cavity laser described in this disclosure has utilityin a large number of optical systems, some of which have been brieflyalluded to in the preceding description. A few representative examplesof those systems are herein now described. A system for SSOCT can employa tunable laser comprising the tunable short-cavity laser describedabove, in combination with a means for splitting tunable radiation fromthe tunable laser to a reference path and a sample path, and an opticaldetector configured to detect an interference signal between lightreflected from said sample and traversing said reference path. Signalprocessing of this interference signal can then be used to reconstructstructural or compositional information about the sample, as iswell-know to those skilled in the art of SSOCT.

A system for optical spectroscopy can employ the tunable short-cavitylaser described, in conjunction with an optical detector, to measure anabsorption, transmission, scattering, or reflection spectrum of asample, which can be a solid, liquid, gas, plasma, or any substance inany state of matter. In addition, the variable tuning speed of thetunable short cavity laser can be used to scan across an opticalspectrum at variable speed, slowing down information rich regions andspeeding up in less-information rich regions, to obtain a desired signalto noise ratio while minimizing measurement time.

The tunable short cavity laser described can, in combination with adispersive optical element, be employed in a system for optical beamsteering. For example, it is well-known that the diffraction angle of agrating is a function of the wavelength of input tunable radiation.Thus, tuning the radiation will scan the diffraction angle and achieveoptical beam steering. Other dispersive elements such as prisms can alsobe employed.

Other optical systems which can employ a short-cavity laser according toan embodiment of the present invention include a distanceinterferometer, where switching between two or more wavelengths can beused to infer distance.

An embodiment of the present invention can also be used to create atunable oscillator, by beating the tunable output of the short-cavitylaser with a fixed wavelength reference laser. This beating can beaccomplished by, for example, an optical detector that responds toincident optical power. If two collinear laser beams impinge on thisdetector, the detector output will oscillate at the difference inoptical frequencies between the two laser beams, provided thatdifference frequency is within the detector bandwidth. As one laser istuned, this difference frequency will also tune, creating a tunableoscillator down-shifted from optical frequencies to lower frequencies.

While the present invention has been described at some length and withsome particularity with respect to the several described embodiments, itis not intended that it should be limited to any such particulars orembodiments or any particular embodiment, but it is to be construed withreferences to the appended claims so as to provide the broadest possibleinterpretation of such claims in view of the prior art and, therefore,to effectively encompass the intended scope of the invention.Furthermore, the foregoing describes the invention in terms ofembodiments foreseen by the inventor for which an enabling descriptionwas available, notwithstanding that insubstantial modifications of theinvention, not presently foreseen, may nonetheless represent equivalentsthereto.

What is claimed is:
 1. A tunable laser operative to emit tunableradiation over an emission wavelength range having a center wavelength,with an output power spectrum over said wavelength range and an averageemission power, said tunable laser comprising: an optical cavityincluding a first and second mirror; a gain region interposed betweensaid first and second mirrors; a source of pump energy, a tuning region;and means for adjusting an optical path length of said tuning region tocreate wavelength tuning of said tunable laser; wherein: a free spectralrange (FSR) of said optical cavity exceeds 5% of said center wavelength;said tunable laser operates in a single longitudinal and transverse modeover said wavelength range; and said pump energy is dynamicallycontrolled during said wavelength tuning.
 2. The tunable laser of claim1, wherein said output power spectrum has a target spectral shape, andsaid dynamically controlled pump energy is controlled to achieve saidtarget spectral shape.
 3. The tunable laser of claim 1, wherein saiddynamically controlled pump energy turns off said tunable radiation overa portion of said wavelength tuning.
 4. The tunable laser of claim 1,wherein said tunable laser is optically pumped and said pump energy isan optical pump.
 5. The tunable laser of claim 1, wherein said tunablelaser is electrically pumped and said pump energy is an electrical drivecurrent.
 6. The tunable laser of claim 2, wherein said spectral shape isGaussian.
 7. The tunable laser of claim 1, further comprising an opticalamplifier.
 8. The tunable laser of claim 7, wherein said pump energypumps said optical amplifier.
 9. The tunable laser of claim 1, whereinsaid pump energy pumps said gain region between said first and secondmirrors.
 10. A tunable laser operative to emit tunable radiation over anemission wavelength range having a center wavelength, with an outputpower spectrum having a target spectral shape over said wavelength rangeand an average emission power, said tunable laser comprising: an opticalcavity including a first and second mirror; a gain region interposedbetween said first and second mirrors; a tuning region; means foradjusting an optical path length of said tuning region to createwavelength tuning of said tunable laser; and an optical amplifier havingan amplified spontaneous emission (ASE) spectrum; wherein: a freespectral range (FSR) of said optical cavity exceeds 5% of said centerwavelength; said tunable laser operates in a single longitudinal andtransverse mode over said wavelength range; and an alignment of said ASEspectrum relative to said emission wavelength range is controlled toachieve said target spectral shape.
 11. The tunable laser of claim 10,wherein said ASE spectrum is blue-shifted relative to said emissionwavelength range.
 12. A system for swept source optical coherencetomography, the system comprising a swept laser source generatingwavelength swept radiation, said swept laser source comprising: thetunable laser of claim 1; means for directing said wavelength-sweptradiation to a reference path and a sample; an optical detector fordetecting an interference signal between light back-reflected from saidsample and light traversing said reference path; and means for signalprocessing of said interference signal to infer structural informationabout said sample.
 13. A system for swept source optical coherencetomography, the system comprising a swept laser source generatingwavelength swept radiation, said swept laser source comprising: thetunable laser of claim 8; means for directing said wavelength-sweptradiation to a reference path and a sample; an optical detector fordetecting an interference signal between light back-reflected from saidsample and light traversing said reference path; and means for signalprocessing of said interference signal to infer structural informationabout said sample.
 14. A system for swept source optical coherencetomography, the system comprising a swept laser source generatingwavelength swept radiation, said swept laser source comprising: thetunable laser of claim 10; means for directing said wavelength-sweptradiation to a reference path and a sample; an optical detector fordetecting an interference signal between light back-reflected from saidsample and light traversing said reference path; and means for signalprocessing of said interference signal to infer structural informationabout said sample.
 15. A method for generating an output power variationversus wavelength over a wavelength range from a tunable laser operativeto emit tunable radiation over an emission wavelength range having acenter wavelength, with an output power spectrum over said wavelengthrange and an average emission power, said tunable laser comprising: anoptical cavity including a first and second mirror; a gain regioninterposed between said first and second mirrors; a tuning region; andmeans for adjusting an optical path length of said tuning region;wherein: a free spectral range (FSR) of said optical cavity exceeds 5%of said center wavelength; and said tunable laser operates in a singlelongitudinal and transverse mode over said wavelength range; the methodcomprising dynamically controlling a pump energy entering into the gainregion versus wavelength during wavelength tuning.
 16. The method ofclaim 15, wherein said output power spectrum has a target spectralshape, and said pump energy is dynamically controlled during saidwavelength tuning to achieve said target spectral shape.
 17. A methodfor generating a desired output power variation versus wavelength over awavelength range from a tunable laser operative to emit tunableradiation over an emission wavelength range having a center wavelength,with an output power spectrum over said wavelength range and an averageemission power, said tunable laser comprising: an optical cavityincluding a first and second mirror; a gain region interposed betweensaid first and second mirrors; a tuning region; and means for adjustingan optical path length of said tuning region; wherein: a free spectralrange (FSR) of said optical cavity exceeds 5% of said center wavelength;and said tunable laser operates in a single longitudinal and transversemode over said wavelength range; the method comprising defining a targetpower variation over the wavelength range, and adjusting a reflectivityspectrum of one of said first and second mirrors until said output powervariation matches said target power variation over the wavelength range.